Outflow water clock

The obvious limirtation of sundials is that they cannot tell the time during the night or when the sun is overcast. Timekeeping with reference to the night sky is one very ancient alternative. Another is to develop mechanical devices capable ot counting off the hours. The most ancient and widely-disseminated device for this purpose is the water clock or clepsydra (‘water stealer’).

There were two basic kinds of water clock: outflow and inflow.

Outflow water clocks, the simplest form, were already know in ancietn Egypt, c. 1500 BC. These clocks consisted simply of a vessel with a narrow outflow spot made to discharge water slowly and at a steady pace. The inside of a container was marked with lines of measurement. Observers were able to tell time by measuring the change in water level against the lines marked on the interior of the vessel. Operators could adjust them to the changing seasons by varying the amount of liquid in the container. A video representation of an ancient Greek device of this very simple kind can be found here. 

One way of compensating for the variable pressure and rate of flow in devices of this kind was to use a graduated parabolic scale, which placed markings for the hours further apart at the top (when the water pressure and rate of flow were higher) and closer together at the bottom (where the pressure and flow rate declined). An animation of this solution can be found here. 

Commentary. Howard Hotson (May 2021)